I want to customise voicemail is such a way to attach an image as a signature on some voice-mail accounts when it sends as an email.
Use the mailcmd option to intercept the mail and post process it.
However, I would advise against doing this, as signatures and letter heads are one of the worst affectations to hit email. All they really do is bloat the mail and increase the risk that anti-spam heuristics will delete the mail.
Signatures are particularly bad. As the recipient knows they have been added mechanically, they know that there is no sincerity attached to them.
Thanks david for your good advice.
But it is company requirement .
Also please check this link related to it.